Positive electrode and method for making the same, and battery using the same

ABSTRACT

A method for making a positive electrode includes the following steps: dispersing a plurality of carbon nanotubes in water, to form a carbon nanotube dispersion; adding an aniline solution into the carbon nanotube dispersion, to form a mixed solution; adding an initiator into the mixed solution, to form a carbon nanotube composite structure preform; freeze-drying the carbon nanotube composite structure preform in a vacuum environment; carbonizing the carbon nanotube composite structure preform in a protective gas after freeze-drying, to form a carbon nanotube composite structure; and adding a positive electrode active material into the carbon nanotube composite structure. The present application also relates to the positive electrode and a battery including the positive electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly-assigned application entitled,“CARBON NANOTUBE COMPOSITE STRUCTURE AND METHOD FOR MAKING THE SAME”,concurrently filed (Atty. Docket No. US74609). Disclosures of theabove-identified applications are incorporated herein by reference.

FIELD

The present application relates to a positive electrode and a method formaking the same, and a battery using the same.

BACKGROUND

Carbon nanotubes can be composed of a number of coaxial cylinders ofgraphite sheets, and have recently attracted a great deal of attentionfor use in different applications, such as field emitters, chemicalsensors, battery, and so on. With the development of lithium/sulfur(Li/S) battery, the application of carbon nanotubes in the Li/S batteryis a research hotspot.

The Li/S battery is a lithium battery with sulfur as the positiveelectrode and lithium as the negative electrode. The working mechanismof the Li/S battery is different from that of the lithium ion battery,the former is the electrochemical mechanism, and the latter is lithiumion intercalation-deintercalation mechanism. The Li/S battery isconsidered to be one of the most promising candidates fornext-generation energy storage devices because of its high theoreticalspecific energy (2600 Whkg⁻¹) and abundant resources. However, the lowsulfur utilization and rapid capacity fading causes the Li/S battery tohave the short cycle lifetime, thereby preventing the Li/S battery fromcommercialization. At present, the carbon nanotubes can be used as acurrent collector of a battery, but the mechanical property, theelectrical property, and the chemical property of the carbon nanotubesare insufficient to solve the above problems.

Therefore, there is room for improvement in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a process flow of a first embodiment of a method for making acarbon nanotube composite structure.

FIG. 2 shows an optical image of the first embodiment of a CNT/PANIfoam.

FIG. 3 shows a scanning electron microscope (SEM) image of the firstembodiment of a CNT foam.

FIG. 4 shows a transmission electron microscope (TEM) image of the firstembodiment of the CNT foam.

FIG. 5 shows a SEM image of the carbon nanotube composite structureprepared by the method of FIG. 1.

FIG. 6 shows a TEM image of the carbon nanotube composite structureprepared by the method of FIG. 1.

FIG. 7 shows an energy dispersive spectroscopy (EDS) image of the carbonnanotube composite structure prepared by the method of FIG. 1.

FIG. 8 shows stress-strain curves of the carbon nanotube compositestructure prepared by the method of FIG. 1 and the CNT foam.

FIG. 9 schematically shows the carbon nanotube composite structureprepared by the method of FIG. 1.

FIG. 10 is a process flow of a second embodiment of a method for makinga positive electrode.

FIG. 11 schematically shows the positive electrode prepared by themethod of FIG. 10.

FIG. 12 schematically shows a third embodiment of a battery.

FIG. 13 shows cyclic voltammetry curves at different cycle numbers ofthe third embodiment of the positive electrode of a first coin battery.

FIG. 14 shows cyclic specific capacity curves at 1 C of the thirdembodiment of the positive electrodes in the first coin battery and asecond coin battery.

FIG. 15 shows charge and discharge voltage-specific capacity curves at0.1 C of the third embodiment of the positive electrodes in the firstcoin battery and a second coin battery.

FIG. 16 shows square root of scan rate-peak current curves of the thirdembodiment of the positive electrodes in the first coin battery and asecond coin battery.

FIG. 17 shows optical images of the third embodiment of an LED batterypack illuminated by a first pouch battery with different bending angles.

FIG. 18 shows cycle specific capacity curves of the third embodiment ofthe first pouch battery with different bending angles.

FIG. 19 shows cycle specific capacity curves of the third embodiment ofa second pouch battery.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. The drawings are not necessarily to scale, andthe proportions of certain parts may be exaggerated to illustratedetails and features better. The description is not to be considered aslimiting the scope of the embodiments described herein.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape or other word that substantially modifies,such that the component need not be exact. For example, substantiallycylindrical means that the object resembles a cylinder, but can have oneor more deviations from a true cylinder. The term “comprising” means“including, but not necessarily limited to”; it specifically indicatesopen-ended inclusion or membership in a so-described combination, group,series and the like.

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

FIG. 1 and FIG. 9 show a method for making a carbon nanotube compositestructure 10 of a first embodiment, and the method includes one or moreof the following steps:

S11, dispersing a plurality of carbon nanotubes 12 in water, to form acarbon nanotube dispersion;

S12, adding an aniline solution to the carbon nanotube dispersion, toform a mixed solution, wherein the aniline solution is formed bydissolving aniline in a solvent;

S13, adding an initiator to the mixed solution to initiatepolymerization of the aniline, to form a carbon nanotube compositestructure preform;

S14, freeze-drying the carbon nanotube composite structure preform in avacuum environment; and

S15, carbonizing the carbon nanotube composite structure preform in aprotective gas after freeze-drying, to form the carbon nanotubecomposite structure 10.

During step S11, the carbon nanotubes 12 may be single-walled,double-walled, multi-walled carbon nanotubes, or their combinations. Thediameters of the carbon nanotubes 12 range from about 20 nanometers (nm)to about 30 nm. In one embodiment, the lengths of the carbon nanotubes12 are greater than 100 microns. In another embodiment, the lengths ofthe carbon nanotubes 12 are greater than 300 microns. In one embodiment,the surface of the carbon nanotube 12 is pure and free of impurities,and has not undergone any chemical modification. The impurities orchemical modification would destroy the bonding force between the carbonnanotubes 12.

The method for making the carbon nanotubes 12 includes the followingsteps: growing a carbon nanotube array including the carbon nanotubes 12on a growth substrate; and separating the carbon nanotubes 12 from thegrowth substrate by a knife or other tool. In one embodiment, the carbonnanotube array is a super-aligned carbon nanotube array. In thesuper-aligned carbon nanotube array, the carbon nanotubes 12substantially have the same length direction, and the lengths of thecarbon nanotubes 12 are greater than 300 microns. The super-alignedcarbon nanotube array is essentially free of impurities such ascarbonaceous or residual catalyst particles.

The water can be pure water. The carbon nanotubes 12 can be dispersed inthe water by ultrasonically agitating or the like. After dispersing thecarbon nanotubes 12 in the water, the carbon nanotubes 12 aresubstantially uniformly distributed in the water to form a flocculentstructure. In the flocculent structure, the carbon nanotubes 12 are notseparated apart from each other, but entangled and attracted to eachother. The flocculent structure has a plurality of pores, and theplurality of pores are filled with water. During the process ofultrasonically agitating, a power of the ultrasonic wave can range fromabout 300 watts (W) to about 1500 W, and a time of ultrasonicallyagitating can range from about 10 minutes to about 60 minutes. In orderto effectively disperse the carbon nanotubes 12 in the water, asurfactant can be added into the water. The type of the surfactant isnot limited, such as, fatty acid glycerides, stearic acid,polyvinylpyrrolidone, or the like. In one embodiment, the power of theultrasonic wave ranges from about 500 W to about 1200 W, and thesurfactant is polyvinylpyrrolidone.

Although the dispersion effect of the carbon nanotubes 12 in an organicsolvent is better than that in water, the freezing point of the organicsolvent is generally lower than −100 degrees Celsius. Thus, it isdifficult to freeze-dry the carbon nanotube composite structure preform.Thus, the carbon nanotubes 12 are dispersed in water so that the poresof the carbon nanotube composite structure preform are filled withwater, thereby facilitating freeze-drying the carbon nanotube compositestructure preform.

During step S12, the solvent can dissolve the aniline, and the solventcan be hydrochloric acid, ethanol, or the like. In one embodiment, theaniline is first dissolved in ultrapure water to form the anilineultrapure water solution, and then hydrochloric acid is added into theaniline ultrapure water solution, to form the aniline solution.

During step S13, the aniline is polymerized under the action of theinitiator to form the polyaniline. The carbon nanotube compositestructure preform includes the carbon nanotubes 12, the initiator, thesolvent, and the water. The polyaniline is attached on the outer surfaceof each carbon nanotube 12. In one embodiment, the carbon nanotubecomposite structure preform has a hydrogel shape. The material of theinitiator is not limited as long as the aniline can be polymerized underthe action of the initiator. The initiator can be ammonium persulphateor the like. In one embodiment, the ammonium persulphate is dissolved inwater to form an ammonium persulphate aqueous solution, and the ammoniumpersulphate aqueous solution is added into the mixed solution.

During step S14, freeze-drying the carbon nanotube composite structurepreform in the vacuum environment includes the following sub-steps:

S141, placing the carbon nanotube composite structure preform into afreeze drier, and rapidly cooling the carbon nanotube compositestructure preform to a temperature lower than −50 degrees Celsius; and

S142, creating a vacuum in the freezer drier and increasing thetemperature of the carbon nanotube composite structure preform to a roomtemperature in gradual stages, wherein a time of drying in differentstages ranges from about 1 hour to about 10 hours, and the vacuum degreein the freeze drier ranges from about 1 Pa to about 10 Pa.

The step of freeze-drying the carbon nanotube composite structurepreform in the vacuum environment can prevent the carbon nanotubecomposite structure preform from collapsing, to form a sponge-likestructure. The carbon nanotube composite structure preform isfreeze-dried to form a CNT/PANI foam, as shown in FIG. 2. The terms“CNT/PANI” represents a composite structure formed by the carbonnanotubes 12 and the polyaniline. The CNT/PANI foam is a sponge. TheCNT/PANI foam includes the carbon nanotubes 12, and each carbon nanotube12 is coated with the polyaniline. In one embodiment, the polyanilinecoats the outer surface of each carbon nanotube 12.

During step S15, the polyaniline and the initiator are carbonized underthe protective gas. The protective gas is an inert gas, and the inertgas can be argon gas or the like. The carbonization temperature rangesfrom about 800 degrees Celsius to about 1200 degrees Celsius. The carbonnanotubes 12 are not carbonized because of the presence of theprotective gas. In one embodiment, the polyaniline and the ammoniumpersulphate (initiator) are carbonized to form a sulfur,nitrogen-codoped carbon (SNC) layer 14. The SNC layer 14 is located onthe outer surface of each carbon nanotube 12. Thus, the carbon nanotubecomposite structure 10 includes the carbon nanotubes 12, and the SNClayer 14 is coated on outer surface of each carbon nanotube 12.

The ammonium persulphate is not only used as the initiator, but alsoused as a dopant source of the nitrogen element and the sulfur element.In process of carbonizing, the ammonium persulphate is chemicallyconverted to the nitrogen element and the sulfur element, and the insitu polymerized polyaniline is chemically converted to nitrogen-dopedcarbon element. The amorphous SNC layer 14 is formed on each carbonnanotube 12, to obtain a coaxial carbon skeleton. Thus, the carbonnanotube composite structure 10 is a solid and stable “bird's nest”carbon frame structure.

Hereinafter, in the first embodiment, a specific example of the methodfor making the carbon nanotube composite structure 10 and a comparativeexample are described.

Specific Example

The 100 mg carbon nanotubes 12 from the super-aligned carbon nanotubearray and 15 mg polyvinylpyrrolidone (PVP K90) are dispersed inultrapure water and effectively sonicated for 30 min, to form an uniformcarbon nanotube dispersion. Then the carbon nanotube dispersion isplaced into a three-necked bottle with ice bath cooling, and magneticstirring the carbon nanotube dispersion immediately. The 150 μL aniline(ANI, J&K Scientific, 98.5%) is dissolved in 20 mL ultrapure water, toform an aniline aqueous solution. The 5 mL HCL (0.1 molL⁻¹) was added inthe aniline aqueous solution, to form an aniline solution. The anilinesolution is slowly added into the carbon nanotube dispersion, andcontinuously stirring for 30 minutes after adding the aniline solution.Then the 50 mL ammonium persulphate aqueous solution (ammoniumpersulphate: 368 mg) is dropwise added under vigorous stirring andnitrogen flow. After stirring for 24 hours, the CNT/PANI hydrogel isformed. The CNT/PANI hydrogel is placed into a petri-dish (diameter: 140mm) and freezing-drying for several days, to form the CNT/PANI foam. Thetemperature of freezing-drying is about −76 degrees Celsius, and thevacuum degree is about 1 Pa. Finally, the CNT/PANI foam is placed in atube furnace and is carbonized at a high temperature under an argonflow, the temperature is slowly increased, and the heating rate is about3 degrees Celsius per minute. The specific process is as follows: thetemperature is raised to 275 degrees Celsius from room temperature, andpre-carbonized for 1 hour; then raised to 900 degrees Celsius, andcompletely carbonized for 3 hours; finally, after cooling to roomtemperature, the carbon nanotube composite structure 10 is formed.

Comparative Example

The 100 mg carbon nanotubes 12 from the super-aligned carbon nanotubearray and 15 mg polyvinylpyrrolidone (PVP K90) are dispersed inultrapure water and effectively sonicated for 30 min, to form theuniform carbon nanotube dispersion. Then the carbon nanotube dispersionis placed into a three-necked bottle with ice bath cooling, and magneticstirring the carbon nanotube dispersion immediately. Then the carbonnanotube dispersion is placed into a petri-dish (diameter: 140 mm) andfreezing-drying for several days, to form a CNT foam. The temperature offreezing-drying is about −76 degrees Celsius, and the vacuum degree isabout 1 Pa.

The comparative example is similar to the specific example of the methodfor making the carbon nanotube composite structure 10 above except thatthe comparative example does not include the steps of adding the anilinesolution and the ammonium persulphate aqueous solution, and also doesnot include the step of carbonization.

FIG. 3 shows a scanning electron microscope (SEM) image of the CNT foamof the comparative example. FIG. 4 shows a transmission electronmicroscope (TEM) image of the CNT foam of the comparative example. FIG.5 shows a SEM image of the carbon nanotube composite structure 10 of thespecific example. FIG. 6 shows a TEM image of the carbon nanotubecomposite structure 10 of the specific example. Comparing FIG. 3 andFIG. 5, and comparing FIG. 4 and FIG. 6, it can be seen that the outersurface of the carbon nanotube 12 is coated with a coating. FIG. 7 is anEDS spectrum of the carbon nanotube composite structure 10. Seen fromFIG. 7, the coating includes carbon (C) element, nitrogen (N) element,and sulfur (S) element. Thus, the coating is the sulfur,nitrogen-codoped carbon (SNC) layer 14. The ratio of the carbon (C)element, nitrogen (N) element, and sulfur (S) element, and the thicknessof the SNC layer 14 can be controlled by the amount of aniline and theinitiator, such as by the mass fraction of each substance andconcentration of the solvent. In one embodiment, the analysis results ofthe X-ray photoelectron spectroscopy (XPS) shows that the atomicpercentage of the carbon (C) element, nitrogen (N) element, and sulfur(S) element in the carbon nanotube composite structure 10 isC:N:S=95.4%:3.2%:1.4%.

Furthermore, seen from FIG. 5 and FIG. 6, the intersections of multiplecarbon nanotubes 12 are bonded by the SNC layer 14, improving thestability of the carbon nanotube composite structure 10 and introducingmore active sites. Moreover, FIG. 5 and FIG. 6 show that the carbonnanotube composite structure 10 has a carbon nanotube network structure.The strong carbon nanotube conductive network structure is essential forhigh sulfur loading and rapid charge transferring. The strong carbonnanotube conductive network structure is capable of mitigating volumeexpansion of the electrode active material during lithiation process,and is capable of withstanding mechanical bending and folding of theelectrode.

FIG. 8 shows the mechanical test results of the carbon nanotubecomposite structure 10 and the CNT foam. Seen from FIG. 8, the strain ofthe carbon nanotube composite structure 10 is almost five times that ofthe CNT foam in the range of 2% relative displacement. Seen from FIG. 8,after the initial displacement, the two curves have the same slope. Thisindicates that the carbon nanotube composite structure 10 changes fromelastic deformation to plastic deformation. This phenomenon can beexplained by the fact: the SNC layer 14 has fastening or bondingfunction at the intersection of multiple carbon nanotubes 12 during theinitial stretching; with further stretching, excessive stress weakensthe fastening or bonding function of the SNC layer 14, causing theelastic deformation to transform into plastic deformation.

Due to the bonding of the SNC layer 14, the ability of carbon nanotubecomposite structure 10 to resist deformation is far greater than theability of CNT foam to resist deformation. The carbon nanotube compositestructure 10 and the CNT foam are respectively cut into strips having awidth of 8 mm to 10 mm and a length greater than 1 cm, and then thestrips are pulled by a mechanical tester (Instron MicroTester 5848). Thetensile rate under a 20 N load is 1% strain rate per minute. The tensiletest shows that the Young's modulus of the carbon nanotube compositestructure 10 is 810.12 MPa, and the Young's modulus of the pure CNT foamis only 106.82 MPa.

Referring to FIG. 9, the carbon nanotube composite structure 10 in thefirst embodiment includes a carbon nanotube network structure and theSNC layer 14. The carbon nanotube network structure includes theplurality of carbon nanotubes 12 entangled to each other. The pluralityof pores is formed between the plurality of carbon nanotubes 12. The SNClayer 14 is coated on the outer surface of the each carbon nanotube 12.In one embodiment, the SNC layer 14 covers almost the entire outersurface of each carbon nanotube 12. The SNC layers 14 are integrated atthe junction of two adjacent carbon nanotubes 12. The intersected carbonnanotubes 12 are bonded together by the SNC layers 14. Thus, themultiple carbon nanotubes 12 are firmly combined so that the carbonnanotube composite structure 10 does not collapsed. When the SNC layer14 is coated on the outer surface of each carbon nanotube 12, the carbonnanotube composite structure 10 still has multiple pores. Since thecarbon nanotube composite structure 10 has multiple pores, the carbonnanotube composite structure 10 has a large specific surface area andgood elasticity, and is a complete elastic body. The SNC layer 14includes three elements, such as carbon (C), nitrogen (N), and sulfur(S). In one embodiment, the SNC layer 14 consists of three elements ofcarbon (C), nitrogen (N), and sulfur (S).

FIG. 10 and FIG. 11 shows a method for making a positive electrode 20 ofa second embodiment, and the method includes one or more of thefollowing steps:

S21, dispersing a plurality of carbon nanotubes 12 in water, to form acarbon nanotube dispersion;

S22, adding an aniline solution to the carbon nanotube dispersion, toform a mixed solution, wherein the aniline solution is formed bydissolving aniline in a solvent;

S23, adding an initiator to the mixed solution to initiatepolymerization of the aniline, to form a carbon nanotube compositestructure preform;

S24, freeze-drying the carbon nanotube composite structure preform in avacuum environment;

S25, carbonizing the carbon nanotube composite structure preform in aprotective gas after freeze-drying, to form the carbon nanotubecomposite structure 10; and

S26, adding a positive electrode active material to the carbon nanotubecomposite structure 10.

The method for making the positive electrode 20 of the second embodimentis similar to the method for making the carbon nanotube compositestructure 10 of the first embodiment above except that the method formaking the positive electrode 20 further includes a step of adding thepositive electrode active material to the carbon nanotube compositestructure 10.

During step S26, adding the positive electrode active material to thecarbon nanotube composite structure 10 includes the following sub-steps:

S261, dissolving the positive electrode active material in an organicsolvent, to form a positive electrode active material solution;

S262, adding the positive electrode active material solution to thecarbon nanotube composite structure 10; and

S263, removing the organic solvent.

During step S261, the type of the positive electrode active material isnot limited. In one embodiment, the positive electrode active materialis a polysulfide, sulfur, or lithium sulfide. The polysulfide can beLi₂S₆ or Li₂S₈. In one embodiment, the positive electrode activematerial is Li₂S₈, and the organic solvent is 1, 2-dimethoxyethane and1, 3-dioxolane mixture.

During step S262, the positive electrode active material solution isdropped into the carbon nanotube composite structure 10. During stepS263, the method for removing the organic solvent is not limited, forexample, the organic solvent is evaporated by heating.

Hereinafter, in the second embodiment, a specific example of the methodfor making the positive electrode 20 and a comparative example aredescribed.

Specific Example

The 100 mg carbon nanotubes 12 from the super-aligned carbon nanotubearray and 15 mg polyvinylpyrrolidone (PVP K90) are dispersed inultrapure water and effectively sonicated for 30 min, to form theuniform carbon nanotube dispersion. Then the carbon nanotube dispersionis placed into a three-necked bottle with ice bath cooling, and magneticstirring the carbon nanotube dispersion immediately. The 150 μL aniline(ANI, J&K Scientific, 98.5%) is dissolved in 20 mL ultrapure water, toform an aniline aqueous solution. The 5 mL HCL (0.1 molL⁻¹) was added inthe aniline aqueous solution, to form an aniline solution. The anilinesolution is slowly added into the carbon nanotube dispersion, andcontinuously stirring for 30 minutes after adding the aniline solution.Then the 50 mL ammonium persulphate aqueous solution (ammoniumpersulphate: 368 mg) is dropwise added under vigorous stirring andnitrogen flow. After stirring for 24 hours, the CNT/PANI hydrogel isformed. The CNT/PANI hydrogel is placed into the petri-dish (diameter:140 mm) and freezing-drying for several days, to form the CNT/PANI foam.The temperature of freezing-drying is about −76 degrees Celsius, and thevacuum degree is about 1 Pa. Finally, the CNT/PANI foam is placed in thetube furnace and is carbonized at the high temperature under the argonflow, the temperature is slowly increased, and a heating rate is about 3degrees Celsius per minute. The specific process is as follows: thetemperature is raised to 275 degrees Celsius from room temperature, andpre-carbonized for 1 hour; then raised to 900 degrees Celsius, andcompletely carbonized for 3 hours; finally, after cooling to roomtemperature, the carbon nanotube composite structure 10 is formed.

The 20 mL 1, 2-dimethoxyethane/1, 3-dioxolane (DME/DOL, volume ratio1:1) is placed in a flask. Then, according to the equationLi₂S+7S→Li₂S₈, the commercial sulfur (1444 mg) and correspondingstoichiometric lithium sulfide powder (Li₂S, 276 mg) are placed in theflask. Then the flask is placed in a glove box that filled with argongas. The solution in the flask is magnetically stirred at 50 degreesCelsius for 12 hours, and the two reactants completely react, to obtain0.3 molL⁻¹ Li₂S₈ solution (positive electrode active material, brown).The carbon nanotube composite structure 10 (diameter: 10 mm, averagemass: 1.6 mg) is dried in a vacuum oven at 50 degrees Celsius for 12hours, to reduce water oxygen adsorption. Then about 22 μL Li₂S₈solution (0.3 M) is dropped into the dried carbon nanotube compositestructure 10, corresponding to a sulfur loading of about 2.2 mgcm⁻².After evaporating the solvent at 50 degrees Celsius to obtain aCNT/SNC/Li₂S₈ three-dimensional positive electrode. For large-areapositive electrode 20 that is suitable for pouch battery, the size canbe 48 mm×48 mm, and the area sulfur loading can be 4.4 mgcm⁻² or 7mgcm⁻².

Comparative Example

The 100 mg carbon nanotubes 12 from the super-aligned carbon nanotubearray and 15 mg polyvinylpyrrolidone (PVP K90) are dispersed inultrapure water and effectively sonicated for 30 min, to form theuniform carbon nanotube dispersion. Then the carbon nanotube dispersionis placed into the three-necked bottle with ice bath cooling, andmagnetic stirring the carbon nanotube dispersion immediately. Then thecarbon nanotube dispersion is placed into the petri-dish (diameter: 140mm) and freezing-drying for several days, to form the CNT foam. Thetemperature of freezing-drying is about −76 degrees Celsius, and thevacuum degree is about 1 Pa.

The 20 mL 1, 2-dimethoxyethane/1, 3-dioxolane (DME/DOL, volume ratio1:1) is placed in the flask. Then, according to the equationLi₂S+7S→Li₂S₈, the commercial sulfur (1444 mg) and correspondingstoichiometric lithium sulfide powder (Li₂S, 276 mg) are placed in theflask. Then the flask is placed in the glove box that filled with argongas. The solution in the flask is magnetically stirred at 50 degreesCelsius for 12 hours, and the two reactants completely react, to obtain0.3 molL⁻¹ Li₂S₈ solution (positive electrode active material, brown).The CNT foam (diameter: 10 mm, average mass: 1.6 mg) is dried in thevacuum oven at 50 degrees Celsius for 12 hours, to reduce water oxygenadsorption. Then about 22 μL Li₂S₈ solution (0.3 M) is dropped into thedried CNT foam. After evaporating the solvent at 50 degrees Celsius toobtain a CNT/Li₂S₈ positive electrode.

The comparative example is similar to the specific example of the methodfor making the positive electrode 20 above except that the comparativeexample does not include the steps of adding the aniline solution andthe ammonium persulphate aqueous solution, and also does not include thestep of carbonization.

Referring to FIG. 11, the positive electrode 20 in the second embodimentincludes a current collector and a positive electrode active material22. The current collector is the carbon nanotube composite structure 10.The current collector includes the SNC layer 14 and the plurality ofcarbon nanotubes 12 entangled to each other, and the SNC layer 14 iscoated on the outer surface of the each carbon nanotube 12. Theintersected carbon nanotubes 12 are bonded together by the SNC layers14. The positive electrode active material 22 is located on a surface ofthe SNC layers 14 away from the carbon nanotube 12. The SNC layers 14 islocated between the carbon nanotube 12 and the positive electrode activematerial 22. The SNC layer 14 has good adsorption property topolysulfide. When the positive electrode active material 22 is thepolysulfide, the sulfur (polysulfide) can be firmly fixed on the carbonnanotube 12.

Referring to FIG. 12, a battery 30 of a third embodiment includes thepositive electrode 20, a negative electrode 32, a separator 34, anelectrolyte 36, and a shell 38. The shell 38 defines a space, and thepositive electrode 20, the negative electrode 32, the separator 34, andthe electrolyte 36 are located in the space. The separator 34 is locatedbetween the positive electrode 20 and the negative electrode 32. Thepositive electrode 20, the separator 34, and the negative electrode 32are stacked in that order, and spaced apart from each other. Theelectrolyte 36 is located between the positive electrode 20, theseparator 34, and the negative electrode 32. The positive electrode 20includes the current collector and the positive electrode activematerial 22. The current collector is the carbon nanotube compositestructure 10. The current collector includes the SNC layer 14 and theplurality of carbon nanotubes 12 entangled to each other, and the SNClayer 14 is coated on the outer surface of the each carbon nanotube 12.The intersected carbon nanotubes 12 are bonded together by the SNClayers 14. The SNC layer 14 is located between the carbon nanotube 12and the positive electrode active material 22.

The positive electrode active material can be Li₂S₆ or Li₂S₈. Thematerial of the negative electrode 32 is not limited, such as lithium,magnesium, zinc, or aluminum. The materials of the separator 34 and theelectrolyte 36 are not limited and can be selected according to thepositive electrode 20 and the negative electrode 32. The battery 30 canbe a lithium-sulfur battery, a magnesium-sulfur battery, a zinc-sulfurbattery, or an aluminum-sulfur battery.

In one embodiment, the battery 30 is a lithium sulfur battery; theseparator 34 is Celgard 2400 modified with a nitrogen-doped carbonnanotube (NCNT), and the Celgard 2400 modified with the nitrogen-dopedcarbon nanotube can form a shuttled polysulfide double layer barrier,thereby further fixing sulfur; the material of the negative electrode 32is lithium; and the electrolyte 36 is formed by dissolving 1 molL⁻¹LiTFSI and 1 wt % LiNO₃ in DME and DOL, and the volume ratio of DME toDOL is 1:1. When the battery 30 is 2025 coin battery, the negativeelectrode 32 is a lithium piece with a diameter of 15.6 mm and athickness of 450 microns. When the battery 30 is pouch battery, thenegative electrode 32 is a lithium foil with a thickness of 100 microns,and the size of the lithium foil needs to match the positive electrode20 being 48 mm×48 mm.

An appropriate amount of electrolyte 36 (calculated based on the ratioof the electrolyte 36 to sulfur) needs to be added to the batterysystem. For the pouch battery, the battery system is allowed to standfor 15 minutes before vacuum sealing, so that the electrolyte 36 issufficiently immersed in the battery 30. In one embodiment, in the coilbattery, the ratio of the electrolyte 36 to sulfur is that electrolyte36:sulfur (volume mass) (E/S)=25:1. In another embodiment, in the pouchbattery, the ratio of the electrolyte 36 to sulfur is that electrolyte36:sulfur (volume mass) (E/S)=12:1, or electrolyte 36:sulfur (volumemass) (E/S)=15:1.

In one embodiment, the carbon nanotube composite structure 10 is cutinto a sheet-like body with a diameter of 10 mm, then the Li₂S₈ solution(corresponding to about 2.2 mgcm⁻² sulfur) is added on the sheet-likebody, and the CNT/SNC/S positive electrode is formed after evaporatingthe solvent; the negative electrode 32 is the lithium piece; theelectrolyte 36 is formed by dissolving 1 molL⁻¹ LiTFSI and 1 wt % LiNO₃in DME and DOL, and the volume ratio of DME to DOL is 1:1; the separator34 is Celgard 2400 modified with the nitrogen-doped carbon nanotube(NCNT); and the positive electrode 20, the negative electrode 32, theelectrolyte 36, and the separator 34 are located in the 2025 batteryshell, to form a first coil battery. A CNT/S positive electrode isprepared by the same method above, and a second coil battery includingthe CNT/S positive electrode is formed. The first coil battery and thesecond coil battery have the same negative electrode 32, the sameseparator 34, the same electrolyte 36, and the same battery shell. Onlypositive electrodes 20 of the first coil battery and the second coilbattery are different. In the first coil battery, the positive electrode20 is the CNT/SNC/S positive electrode. In the second coil battery, thepositive electrode 20 is the CNT/S positive electrode. For comparing theperformances of the CNT/SNC/S positive electrode and CNT/S positiveelectrode, various characterization tests of the first coil battery andthe second coil battery are performed.

FIG. 13 shows cyclic voltammetry curves at different cycle numbers ofCNT/SNC/S positive electrode of the first coin battery. Seen from FIG.13, the cyclic voltammetry curves of the CNT/SNC/S positive electrode atthe 20th cycle and the 1th cycle are almost the same, indicating theCNT/SNC/S positive electrode has good cycle performance. FIG. 14 showscyclic specific capacity curves at 1 C of the CNT/SNC/S positiveelectrode in the first coin battery and the CNT/S positive electrode ofthe second coin battery. Seen from FIG. 14, the cyclic specific capacityof the CNT/SNC/S positive electrode is greater than the cyclic specificcapacity of the CNT/S positive electrode in different cycle number,indicating the utilization rate of the positive electrode activematerial of the CNT/SNC/S positive electrode is greater than theutilization rate of the positive electrode active material of the CNT/Spositive electrode.

FIG. 15 shows charge and discharge voltage-capacity curves at 0.1 C ofthe first coin battery and the second coin battery. Seen from FIG. 15,the activation barrier of the CNT/SNC/S positive electrode is lower thanthe activation barrier of the CNT/S positive electrode, indicating theSNC layer 14 has a catalytic effect to the conversion of the dischargeproduct lithium sulfide. The conversion of the discharge product lithiumsulfide is catalyzed, improving the electrochemical reaction rate of thebattery 30, thereby reducing the capacity fading. The capacity fading isreduced, thereby prolonging the cycle life of the battery 30. Thus, theproblem of short cycle life caused by rapid capacity fading can besolved.

FIG. 16 shows a comparative diagram of lithium ion diffusion coefficientanalysis of the positive electrodes in the first coin battery and in thesecond coin battery. The absorption spectrum of FIG. 16 shows that thecharacteristic absorption peak corresponding to the sulfur substancesharply drops after standing for 3 hours, indicating the SNC layer 14has good adsorption to polysulfide, and can fix sulfur on the carbonnanotubes 12, thereby enhancing the sulfur-loading effect of the battery30. Referring again to FIG. 14, the SNC layer 14 coated on the carbonnanotubes 12 enhances the sulfur-loading effect of the battery 30, thusthe utilization rate of the positive active material (sulfur) in theCNT/SNC/S positive electrode is higher than that of the positive activematerial (sulfur) in the CNT/S positive electrode.

In another embodiment, the carbon nanotube composite structure 10 is cutinto a sheet-like body of 48 mm×48 mm, then the Li₂S₈ solution(corresponding to about 4.4 mgcm⁻² sulfur) is added on the sheet-likebody, and the CNT/SNC/S positive electrode is formed after evaporatingthe solvent; the negative electrode 32 is the lithium piece; theelectrolyte 36 is formed by dissolving 1 molL⁻¹ LiTFSI and 1 wt % LiNO₃in DME and DOL, and the volume ratio of DME to DOL is 1:1; the separator34 is Celgard 2400 modified with the nitrogen-doped carbon nanotube(NCNT); and the positive electrode 20, the negative electrode 32, theelectrolyte 36, and the separator 34 are located in an aluminum shell,to form a first pouch battery.

FIG. 17 shows optical images of an LED battery pack illuminated by thefirst pouch battery with different bending angles, wherein the insetfigure shows the positive electrode of the first pouch battery. Seenfrom FIG. 17, when the first pouch battery is bent at 0°, 45°, 135°, or180°, the first pouch battery can illuminate the LED battery pack. Thus,the first pouch battery has good flexibility, and any bend to the firstpouch battery will not affect its use. FIG. 18 shows cycle specificcapacity curves of the first pouch battery with different bendingangles. Seen from FIG. 18, the first pouch battery has good charge anddischarge performance at different bend angles.

In yet another embodiment, the carbon nanotube composite structure 10 iscut into the sheet-like body of 48 mm×48 mm, then the Li₂S₈ solution(corresponding to about 7 mgcm⁻² sulfur) is added on the sheet-likebody, and the CNT/SNC/S positive electrode is formed after evaporatingthe solvent; the negative electrode 32 is the lithium piece; theelectrolyte 36 is formed by dissolving 1 molL⁻¹ LiTFSI and 1 wt % LiNO₃in DME and DOL, and the volume ratio of DME to DOL is 1:1; the ratio ofthe electrolyte 36 to sulfur is that electrolyte 36:sulfur (volume mass)(E/S)=12:1, alternatively the ratio of the electrolyte 36 to sulfur isthat electrolyte 36:sulfur (volume mass) (E/S)=15:1; the separator 34 isCelgard 2400 modified with the nitrogen-doped carbon nanotube (NCNT);and the positive electrode 20, the negative electrode 32, theelectrolyte 36, and the separator 34 are located in the aluminum shell,to form a second pouch battery. FIG. 19 shows the cycle specificcapacity curves of the second pouch battery. Seen from FIG. 19, thesecond pouch battery has good charge and discharge performance.

In summary, the carbon nanotube composite structure 10 and the methodfor making the same have the following advantages: 1) the SNC layer 14is coated on each carbon nanotube 12 to form a coaxial carbon skeleton,the intersections of multiple carbon nanotubes 12 are bonded by the SNClayer 14, improving the stability of the carbon nanotube compositestructure 10. Thus, the Young's modulus of the carbon nanotube compositestructure 10 is 810.12 MPa, and the Young's modulus of the CNT foamwithout the SNC layer 14 is only 106.82 MPa; 2) the intersections ofmultiple carbon nanotubes 12 are bonded by the SNC layer 14, therebyintroducing more active sites; 3) the carbon nanotube compositestructure 10 has a carbon nanotube network structure, the conductivecarbon nanotube network structure is capable of mitigating volumeexpansion of the electrode active material during lithiation process,and is capable of withstanding mechanical bending and folding of theelectrode; 4) the SNC layer 14 has the catalytic effect to theconversion of the discharge product lithium sulfide, improving theelectrochemical reaction rate of the battery 30 and reducing thecapacity fading. Thus the cycle life of the battery 30 is prolonged; 5)the SNC layer 14 has good adsorption to polysulfides, and can fix sulfuron the carbon nanotubes 12, thereby enhancing the sulfur-loading effectof the battery 30. Thus the utilization rate of the active material(sulfur) is improved; 6) the flexible carbon nanotube compositestructure 10 of the battery 30 can make the battery 30 have goodflexibility, and any bending does not affect its use. In addition, thebattery 30 still has good charge and discharge performance underdifferent bending angles.

The embodiments shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, including inmatters of shape, size and arrangement of the parts within theprinciples of the present disclosure up to, and including, the fullextent established by the broad general meaning of the terms used in theclaims.

Additionally, it is also to be understood that the above description andthe claims drawn to a method may comprise some indication in referenceto certain steps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making a positive electrode, themethod comprising: dispersing a plurality of carbon nanotubes in waterto form a carbon nanotube dispersion; adding an aniline solution intothe carbon nanotube dispersion to form a mixed solution, wherein theaniline solution comprises an aniline; adding an initiator into themixed solution to form a carbon nanotube composite structure preform;freeze-drying the carbon nanotube composite structure preform in avacuum environment; carbonizing the carbon nanotube composite structurepreform in a protective gas after freeze-drying, to form a carbonnanotube composite structure; and adding a positive electrode activematerial to the carbon nanotube composite structure.
 2. The method ofclaim 1, wherein adding the initiator into the mixed solution comprisesadding an ammonium persulphate water solution to the mixed solution. 3.The method of claim 1, wherein the initiator is an ammonium persulphate,and the aniline is polymerized to form a polyaniline under the action ofthe initiator; and during carbonizing the carbon nanotube compositestructure preform in the protective gas, the ammonium persulphate isconverted to nitrogen elements and sulfur elements, and the polyanilineis converted to nitrogen-doped carbon elements.
 4. The method of claim3, wherein during carbonizing the carbon nanotube composite structurepreform in the protective gas, the ammonium persulphate and thepolyaniline are converted to form a sulfur, nitrogen-codoped carbonlayer.
 5. The method of claim 1, wherein freeze-drying the carbonnanotube composite structure preform in the vacuum environmentcomprises: placing the carbon nanotube composite structure preform intoa freeze drier, and cooling the carbon nanotube composite structurepreform to a temperature lower than −50 degrees Celsius; and creating avacuum environment in the freezer drier and increasing the temperatureof the carbon nanotube composite structure preform to a room temperaturein stages, wherein a time period of drying in each of the stages rangesfrom about 1 hour to about 10 hours, and the vacuum environment in thefreeze drier ranges from about 1 Pa to about 10 Pa.
 6. The method ofclaim 1, wherein the freeze-drying of the carbon nanotube compositestructure preform is carried out at a temperature of freezing-dryingabout −76 degrees Celsius, and in a vacuum environment of about 1 Pa. 7.The method of claim 1, wherein the carbonizing of the carbon nanotubecomposite structure preform is carried out at a carbonizationtemperature ranging from about 800 degrees Celsius to about 1200 degreesCelsius.
 8. The method of claim 1, wherein adding the positive electrodeactive material to the carbon nanotube composite structure comprises:dissolving the positive electrode active material in an organic solventto form a positive electrode active material solution; adding thepositive electrode active material solution to the carbon nanotubecomposite structure; and removing the organic solvent.
 9. The method ofclaim 8, wherein the positive electrode active material is Li₂S₆ orLi₂S₈, and the organic solvent is 1, 2-dimethoxyethane and 1,3-dioxolane mixture.
 10. A positive electrode, comprising: a positiveelectrode active material; and a current collector, wherein the currentcollector comprises a carbon nanotube network structure and a sulfur,nitrogen-codoped carbon layer, the carbon nanotube network structurecomprises a plurality of carbon nanotubes entangled to each other, thesulfur, nitrogen-codoped carbon layer is coated on outer surfaces of theplurality of carbon nanotubes and intersections of the plurality ofcarbon nanotubes, and the plurality of carbon nanotubes are bondedtogether at the intersections by the sulfur, nitrogen-codoped carbonlayer.
 11. The positive electrode of claim 10, wherein the positiveelectrode active material is located on a surface of the sulfur,nitrogen-codoped carbon layer away from the plurality of carbonnanotubes.
 12. The positive electrode of claim 10, wherein the sulfur,nitrogen-codoped carbon layer is located between the plurality of carbonnanotubes and the positive electrode active material.
 13. The positiveelectrode of claim 10, wherein the positive electrode active material isa polysulfide.
 14. The positive electrode of claim 10, wherein thesulfur, nitrogen-codoped carbon layer comprises nitrogen elements,sulfur elements, and carbon elements.
 15. A battery, comprising: anegative electrode, a separator, an electrolyte, a shell, and a positiveelectrode; wherein the positive electrode comprises: a positiveelectrode active material; and a current collector, wherein the currentcollector comprises a carbon nanotube network structure and a sulfur,nitrogen-codoped carbon layer, the carbon nanotube network structurecomprises a plurality of carbon nanotubes entangled to each other, thesulfur, nitrogen-codoped carbon layer is coated on outer surfaces of theplurality of carbon nanotubes and intersections of the plurality ofcarbon nanotubes, and the plurality of carbon nanotubes are bondedtogether at the intersections by the sulfur, nitrogen-codoped carbonlayer.
 16. The battery of claim 15, wherein the positive electrodeactive material is located on a surface of the sulfur, nitrogen-codopedcarbon layer away from the plurality of carbon nanotubes.
 17. Thebattery of claim 15, wherein the sulfur, nitrogen-codoped carbon layeris located between the plurality of carbon nanotubes and the positiveelectrode active material.
 18. The battery of claim 15, wherein thepositive electrode active material is a polysulfide.
 19. The battery ofclaim 15, wherein the sulfur, nitrogen-codoped carbon layer comprisesnitrogen elements, sulfur elements, and carbon elements.
 20. The batteryof claim 15, wherein a material of the negative electrode is lithium,magnesium, zinc, or aluminum.